In general, the suspension polymerization of vinyl monomer is conducted in an aqueous medium containing a catalyst, a suspending or dispersing agent, and other additives, if necessary or desirable. In carrying out the reaction the ingredients are placed in a pressureproof reaction vessel which is equipped with a stirrer or agitator and a cooling jacket, usually cooled with water. The contents of the reactor are vigorously stirred and the temperature thereof maintained in the reaction temperature range. Since the polymerization reaction is exothermic, the cooling jacket is operated to remove the heat generated by the polymerization reaction.
However, in the suspension polymerization of vinyl halides, such as vinyl chloride, for example, the rate of polymerization rises rapidly when the reaction enters its middle or latter stage. This is primarily due to the insolubility of the polymer in the monomer(s) and causes the increased generation of heat, in the latter stages of the polymerization reaction, beyond the capacity of the cooling jacket. As a consequence, temperature control in the reactor is lost. Also, as industry uses larger and larger reactors, the surface area of the reactor does not increase in proportion to the volume of the reactor. Thus, the heat transfer area in the cooling jacket becomes insufficient to cope with productive operation on a larger scale.
In order to remedy the problem of cooling the polymerization mixture effectively in large reactors, the same have been equipped with reflux condensers. These condensers are usually of the multitubular type mounted on the top of the reactor. The vapor, including the monomer(s), from the reaction mixture rise into the condenser and return to the mixture as a cooled condensate thus helping to cool the polymerization mixture. However, the mere installation of a reflux condenser does not solve the problem completely. This is because polymer buildup occurs in the condenser which tends to block the condenser which, of course, negates its function, namely, heat transfer.
The initial limiting factor encountered when using monomer reflux cooling is the increase in the volume of the polymerization mixture or charge due to the vapor trapped therein. The polymer being formed tends to be carried to the top of the charge by the rising vapor bubbles and to remain there. Segregation of the charge in this way occurs because not enough agitator power is available to rewet the polymer suspension and pull it back into the lower part of the reactor. Another reason for the increase in the volume of the polymerization mixture or charge is that a basic increase in the viscosity of the suspension occurs due to a change in the interaction between the polymer particles of the suspension. The change in viscosity usually takes place over about a five percent conversion period when about 60% conversion is reached and results in rapid expansion of the charge.
As the vapor bubbles break through the surface of the reaction mixture considerable splashing results causing the reaction mixture to be carried up into the condenser where undesirable polymer buildup occurs. Reducing the size of the charge so as to provide more space between the surface of the reaction mixture and the entrance to the condenser does not entirely solve the problem since it defeats the purpose of using large reactors by reducing the productive capacity thereof. Accordingly, there is a strong need in the art to eliminate, or substantially reduce, charge expansion and foaming during suspension polymerization in large reactors equipped with external cooling means, such as a reflux condenser.